Fix Cargo manual_readme warning

[Rayan-and-beyond]

Fixes #606.

Removes the explicit package.readme = "README.md" entry from the v1 manifest. Cargo already infers the same README, so this removes the nightly cargo::manual_readme warning without changing the package metadata.

Validation:

• cargo +nightly check: reproduced the warning before the change; no manual_readme warning after it
• Cargo metadata still reports README.md as the package readme
• cargo test: 64 unit tests, 1 integration test, and 14 doctests passed
• cargo +nightly test --all-features: 74 unit tests, 1 integration test, and 19 doctests passed
• git diff --check passed

[alejandro-vaz]

thanks for contributing